1. Field of the Invention
The present invention relates to an image forming apparatus, an apparatus for supplying image data to the image forming apparatus, and a method of interfacing two apparatuses.
2. Related Background Art
FIG. 1 is a schematic diagram showing the structure of an electrophotographic printer which is one example of conventional image forming apparatuses. Referring to FIG. 1, reference numeral 100 represents a photosensitive drum used as an electrostatic latent image storing member. A charger roller 101 is mounted over the photosensitive drum 100 for uniformly charging the surface of the drum 100 in contact therewith. A light emission means applies a light beam 104 to the surface of the photosensitive drum 100 at the charged surface downstream, in the rotation direction, of the contact position with the charger roller 101. The light emission means is constituted by a semiconductor laser 105 for emitting the light beam 104, a scanner 106 for scanning the light beam 104 along the surface of the drum 100, and an optical lens 107 for adjusting the light beam to focus it on the surface of the drum 100 and form a light spot thereon. The light beam 104 is modulated with image data to form an electrostatic latent image on the drum surface. The latent image is developed by a developer 102 to form a toner image, the developer being mounted in contact with the photosensitive drum 100 further downstream, in the rotation direction, of the position where the light beam 104 is applied.
The toner image is transferred on a sheet of transfer paper P by a transfer roller 103 mounted under the photosensitive drum in contact therewith. Paper sheets P are stacked in a paper stacker 108 disposed in front (at the right in FIG. 1) of the photosensitive drum 100. A paper feed roller 109 is disposed at the end portion of the paper stacker 108, facing the left side toward a transport path. Although a single paper stacker is shown in FIG. 1, paper stackers for different paper sizes may be mounted. Between the paper feed roller 109 and transfer roller 103, a registration roller 111 is mounted which corrects a slant transport of the paper sheet P and synchronizes the image formation on the photosensitive drum 100 with the paper transport, to thereby transport the paper sheet P to the transfer position at a predetermined timing. A registration sensor 110 is disposed between the registration roller 111 and paper feed roller 109 to detect a presence of the paper sheet P.
The paper sheet with the toner image still not developed is further transported to a fixing device at the back (at the left in FIG. 1) of the photosensitive drum 100. The fixing device is constituted by a fixing roller 112 with a fixing device (not shown) being embedded therein and a pressure roller 113 for pressing the fixing roller 112. The toner image on the paper sheet still not developed is developed by pressing and heating the paper sheet P transported from the transfer area with the fixing roller 112 and pressure roller 113. An ejecting paper sensor 114 is mounted at the back of the fixing area, the sensor detecting a paper sheet transported from the fixing area. A paper ejecting roller 115 is mounted at the back of the ejecting paper sensor 114 to eject the developed paper sheet P.
Next, a control unit of the electrophotographic printer having the above mechanism will be described with reference to FIG. 2. Referring to FIG. 2, reference numeral 200 represents a host computer at the outside of the electrophotographic printer. The host computer 200 converts image code data entered by a user into parallel or serial data and sends it via a communication line 201 to a controller 202. The controller 202 develops the code data into image data to be transmitted to a printer engine 220, and requests to start printing or paper pre-feeding by sending a command to a printer control unit 203 in the engine 220 or by reading internal data supplied from the printer control unit 203 as statuses. The controller 202 also controls a sync signal for synchronizing an image output timing with a paper transport in the printer. This controller 202 may be installed in the printer or in the host computer.
The controller 202 is connected to an operation panel 204 from which a user sets various printer modes (e. g., a margin area of an image). The operation panel is usually used in an off-line state (disconnected from the communication line to the host computer). The controller 202 connected to the host computer 200 and operation panel 204 transmits data to and from the printer control unit 203 as described above. In order to perform a drive/stop timing control of each mechanical part shown in FIG. 1 and read data supplied from each sensor, the printer control unit 203 is connected to a transporting system drive unit 205, a high voltage drive unit 206, an optical system drive unit 207, a fixing heater control unit 208, and to a sensor input unit 209.
The transporting system drive unit 205 controls a drive/stop of various motors 210 and various rollers 211 and the high voltage drive unit 206 controls a drive/stop of a charger 212, a fixing device 213, and a transfer unit 214, respectively in response to an instruction from the printer control unit 204. The optical system drive unit 207 controls a drive/stop of a laser 215 and a scanner 216 and the fixing heater control unit 208 controls a drive/stop of a fixing heater 217, respectively in response to an instruction from the printer control unit 204. The sensor input unit 209 reads data from a registration sensor 218, an ejecting paper sensor 219, and a paper size sensor 219a, and supplies the data to the printer control unit 204.
Next, signals transferred between the printer controller 202 and printer (engine) control unit 203 will be described.
Signals to be transferred between the printer controller 202 and engine control unit 203 are as in the following. A signal /CPRDY indicates a communication ready state between the printer controller 202 and engine 220. A signal /PPRDY indicates a communication ready state between the engine control unit 203 and printer controller 202. A signal /RDY indicates a stand-by state allowing the engine control unit 203 to start printing. A signal /PRNT is a print request signal to be issued from the printer controller 202 to the engine control unit 203. A signal /VSREQ is a request signal for a vertical sync signal to be issued by the engine control unit 203 to the printer controller 202. A signal /HSREQ is a request signal for a horizontal sync signal to be issued by the engine control unit 203 to the printer controller 202. A signal /BD is a horizontal sync signal to be outputted from the engine control unit 203 to the printer controller 202. A signal /SCLK is a sync clock signal for serial communication. A signal /CMD is a command signal to be issued from the printer controller 202 to the engine control unit 203. A signal /CBSY is a strobe signal for a command output. A signal /STS indicates a status on the engine to be issued from the printer controller 202 in response to a command. A signal /SBSY is a signal used for outputting a status. Timings of these signals in serial communications are illustrated in FIG. 3.
When a command /CMD is outputted from the printer controller 202, a corresponding status /STS signal is outputted from the engine control unit 203. The sync clock /SCLK is outputted from the printer controller 101. It is assumed that serial communications are performed in the unit of 8-bit length and LSB is used as the parity bit.
Statuses of the engine control unit 203 in serial communications are illustrated in FIGS. 4A to 4C. STATUS0 is a basic status and indicates the fundamental state of the engine 220. STATUS1 and STATUS2 indicate the details when a "call" bit is "1". STATUS3 indicates the details of a misprint when a "misprint" bit of STATUS0 is "1".
STATUS4 and STATUS5 indicate a paper size of a standard cassette and an optional cassette (both not shown). STATUS6 indicates a presence or absence of a paper sheet in each paper feeding unit. STATUS7 indicates a presence or absence of a paper feed function of paper feeding units including an optional cassette. STATUS8 indicates the contents of warning.
In a conventional control using the above statuses, the conditions that the signal /RDY becomes "FALSE", i.e., the printer is not in the stand-by state, are satisfied only when one of bits constituting STATUS1 and STATUS2 becomes "1" and the "call" bit of STATUS0 becomes "1", or when one of bits constituting STATUS3 becomes "1" and the "misprint" bit of STATUS0 becomes "1". Therefore, in order for the printer controller 202 to detect a change in STATUS4 to STATUS8 not directly associated with the signal /RDY, the printer controller 202 is required to always read STATUS4 to STATUS8.
However, although the printer controller 202 is required to periodically monitor these statuses such as paper sizes and a presence or absence of a paper sheet on a paper feeding port presently not designated, other than the data representative of the printer status, particularly the printer ready status, these statuses are not so often changed under the circumstances of general users. Therefore, a conventional control is inefficient.
Still further, in the above-described printer system, the printer controller 202 is required to always read these statuses not directly associated with the signal /RDY, so that a process load on the printer controller 202 increases.